1. Field of the Disclosure
The present disclosure relates to a laser apparatus and an extreme ultraviolet light source apparatus.
2. Description of the Related Art
In recent years, along with a progress in miniaturization of semiconductor device, miniaturization of transcription pattern used in photolithography in a semiconductor process has developed rapidly. In the next generation, microfabrication to the extent of 70 nm to 45 nm, or even to the extent of 32 nm and beyond will be required. Therefore, in order to comply with the demand of microfabrication to the extent of 32 nm and beyond, development of such exposure apparatus combining an extreme ultraviolet (EUV) light source for a wavelength of about 13 nm and a reduced projection reflective optics is expected.
As the EUV light source, there are three possible types, which are a laser produced plasma (LPP) light source using plasma generated by irradiating a target with a laser beam, a discharge produced plasma (DPP) light source using plasma generated by electrical discharge, and a synchrotron radiation (SR) light source using orbital radiant light. Among these light sources, the LPP light source has such advantages that luminance can be made extremely high as close to the black-body radiation because plasma density can be made higher compared with the DPP light source and the SR light source. Among these light sources, the LPP light source has such advantages that luminance can be made extremely high as close to the black-body radiation because plasma density can be made higher compared with the DPP light source and the SR light source. Furthermore, the LPP light source has such advantages that there is no construction such as electrode around a light source because the light source is a point light source with nearly isotropic angular distributions, and therefore extremely wide collecting solid angle can be acquired, and so on. Accordingly, the LPP light source having such advantages is expected as a light source for EUV lithography which requires more than several dozen to several hundred watt power.
In the EUV light source apparatus with the LPP system, as disclosed by US Patent Application Publication No. 2008/0149862, firstly, a target material supplied inside a vacuum chamber is excited by irradiation with a laser light and thus be turned into plasma. Then, a light with various wavelength components including an EUV light is emitted from the generated plasma. Then, the EUV light source apparatus focuses the EUV light on a predetermined point by reflecting the EUV light using an EUV collector mirror which selectively reflects an EUV light with a specific wavelength, e.g. a 13.5 nm wavelength component. The reflected EUV light is inputted to an exposure apparatus. On a reflective surface of the EUV collector mirror, a multilayer coating (Mo/Si multilayer coating) with a structure in that thin coating of molybdenum (Mo) and thin coating of silicon (Si) are alternately stacked, for instance, is formed. The multilayer coating exhibits a high reflectance ratio (of about 60% to 70%) with respect to the EUV light with a 13.5 nm wavelength.
Here, a CO2 pulse laser with high repetition (100 kHz) and high power (10 kW) as being required by an EUV light source apparatus, as it stands, cannot be realized by a TEA CO2 laser. According to US Patent Application Publication No. 2008/0149862, a pulse light is generated from an output light having outputted from a master oscillator (MO) of a driver laser by use of a high-speed shutter, and then multistage-amplified by a CO2 gas amplifier being a power amplifier (PA) to be emitted to a target in a way focusing on the target. Thereby, the target turns into plasma, and an EUV light is emitted from the plasma. In this arrangement, the high-speed shutter located on a beam line of the driver laser is controlled based on a measuring result obtained by detecting the emitted EUV light. As a result, a pulse energy of the driver laser is feedback-controlled so that the energy of the EUV light is adjusted to a desired value. Here, the MO and the PA excite CO2 gas being an amplifiable agent by high-frequency discharge.